Wireless radio frequency (RF) communication networks are becoming ubiquitous. Furthermore, the number of devices being connected to wireless networks is growing at a tremendous rate, as the diversity of types of wireless devices being deployed is also increasing. Devices having wireless RF interfaces range from tiny sensors deployed in rain forests (e.g., to track environmental conditions over time), to smart phones, to laptop computers, to RFID tags for tracking products shipped around the world. As more and more applications of wireless network technologies emerge, so too do situations where wireless connections and networks are needed, but for which wireless RF transmission is not possible. The inventive concept describes a wireless ultrasonic communication system that enables communication into and out of RF-impervious enclosures and structures.
Applications of wireless networks that require connections to devices that may reside inside RF-impervious enclosures, including metal enclosures, are of increasing importance. One example application where this occurs is in the tracking of products and/or environmental conditions inside a sealed steel shipping container. For reasons involving security and privacy, the doors of shipping containers in transit cannot be opened. Furthermore, holes or similar modifications may not be made to these standardized shipping containers for reasons of structural integrity and security, and protection of the items within the sealed shipping container.
Much international shipping is carried in standardized shipping containers, which can be transferred by ship, truck, or rail. It is of great importance to both shippers and security professionals to gather information about the contents, location, and environmental conditions within such shipping containers. Inspecting and tracking the contents of stacked shipping containers in transit is problematic. In a shipyard, only the doors of some shipping containers at the bottom of a stack may be accessed. In a container ship, almost none of the shipping container doors are accessible. Even when accessing the door of a shipping container is possible, opening a shipping container door that is in transit may violate international security policy and regulations.
The inside of shipping containers can be equipped with electronic devices capable of storing and collecting a variety of data. For example, an electronic file containing the contents of a shipping container can be stored on a device within a shipping container. Furthermore, environmental conditions such as temperature and acceleration can be measured with sensors, and stored electronically within a shipping container. Standard wireless radio frequency (RF) communication technology cannot be used to retrieve data from a closed shipping container because RF transmissions cannot penetrate the metal surfaces of a shipping container. RF signals are reflected by metal surfaces analogous to the way light is reflected by mirrors. As a result, transmitting digital information using RF networking technology is not possible when attempting to transmit through and/or around stacks of shipping containers. Boring holes in a shipping container for the purpose of establishing direct wired connections to electronic data inside a shipping container is not feasible because such interfaces are not standard and would be difficult to maintain/operate if there ever became such a standard. Furthermore, doing so would generally compromise the structural integrity of the shipping container and alter the standardized structure. The inability to read electronic information stored inside the shipping container (e.g., RFIDs, sensor readings, electronically stored manifests) severely limits the efficiency and effectiveness of security professionals and inspectors in performing their duties. It also limits the ability of commercial shipping companies in tracking and monitoring the status of packages inside shipping containers in transit, as well as monitoring the environmental conditions inside a shipping container.
Some shipping containers are not entirely constructed of metal. For example, a floor of shipping container is provided with metal joists, but may have a plywood bottom. Even though RF transmissions may pass through plywood material, significant problems exist when trying to communicate to the interior of the shipping container using RF transmissions. This is because the bottom of the shipping container is typically shielded by a top metallic surface of an adjacent shipping container upon which it is stacked, or a ground plane (such as the Earth, or a metallic floor of a ship) upon which the shipping container is positioned.
Shipping containers are typically twenty or forty feet long, eight feet wide, and eight and one-half feet tall. They are constructed with four structural steel corner posts terminated at the top and the bottom in cast steel corner blocks. Shipping containers can be stacked up to twelve high, resulting in a towering structure the height of a ten story building. Expansive grids of stacked shipping containers are commonly found in shipyards and deep within the hulls of huge container ships. Shipping containers are positioned and stacked with massive, precision cranes. To make efficient use of limited space, adjacent stacks of shipping containers are typically positioned very close together in shipyards. On container ships, adjacent stacks of shipping containers are often placed in contact with one another and/or held in place with locking pins and/or bracing straps to provide stability. When stacked, the weight of the shipping containers and their cargo is carried by the continuous columns of corner posts at each corner of the stack. For structural and security reasons, the doors of a shipping container must not be opened in transit, and holes may not be drilled through a shipping container's walls.
Closed shipping containers act as Faraday cages, which preclude the use of traditional radio frequency (RF) communication schemes for transmitting data through these metal structures. Traditional radio frequency communication schemes are not capable of transmitting signals through metal enclosures. Ultrasound presents a promising alternative. Steel is an excellent conduit of ultrasonic energy, and ultrasound transmission requires no compromise of the shipping container's structural or security integrity.
Previous researchers have investigated ultrasonic communication in other applications. A patent application [1] exists, for example, for an application involving sensor communication within an airframe; sensor communication along the surface of a single shipping container is also mentioned. However, this prior research: (i) does not consider communication from within a shipping container to outside a shipping container; (ii) does not consider communication among stacked shipping containers; (iii) does not consider two way communication; and (iv) did not proceed to the system prototyping stage.
Research was done at Oak Ridge National Laboratories in 1993 and further in 1999 regarding ultrasonic communications [2]. These researchers successfully built a demonstration system to communicate using ultrasound in air, and also carried out experiments in various types of pipes that might be found in a typical building water supply. The demonstration system built as part of this project consisted of three work boxes and a laptop running LabView Virtual Instruments at each end of the communication channel.
Other research on underwater communication with autonomous underwater vehicles (AUVs) was conducted at Florida Atlantic University in 1996 [3]. Their research involved using multi-tone frequency shift keying (MFSK) and differential phase shift keying (DPSK) to communicate in shallow water.
The most mature applications involving ultrasonics do not involve ultrasonic communications, but instead originate from the fields of non-destructive testing and medical imaging. In nondestructive testing, the propagation of high frequency (several MHz) ultrasound is transmitted through various materials for the detection of material discontinuities such as flaws or cracks [4]. In medical applications, ultrasonic reflections are the basis of forming images. Although there is an extensive body of data related to these types of applications, the high-frequency (and high-resolution) transducers designed to operate in these regimes are costly, and therefore may be impractical for the types of application domains where ultrasonic communications would be employed.
Another application involving the problem of transmitting information through an RF-impervious enclosure arises in oil and gas applications, including downhole drilling and extraction. For reasons involving human and environmental safety and protection, and prevention of damage to various components of the drilling rig, it is important to monitor pressure, and possibly other parameters, such as temperature, using sensors located inside highly pressurized vessels and enclosures, including blowout preventers (BOPs). These highly pressurized enclosures are enormous in size, typically constructed with extremely thick, strong steel components. Due to the extreme conditions and pressures present inside these enclosures, it is not possible or practical to consider boring holes through the walls of these structures with the purpose of passing wiring for making connections between the inside and outside of the enclosure.
In the field of oil and gas exploration and extraction, for reasons of human and environmental safety and prevention of damage to components of the rig, it is increasingly desirable to be able to monitor pressure and/or other sensor readings taken from within the enclosure associated with a blowout preventer (BOP) or a BOP stack. The goal is to transmit pressure measurement readings taken inside a BOP enclosure (or other associated enclosure and/or pipe) to outside the BOP without compromising the structural integrity of the BOP itself (e.g., without boring holes in the thick steel walls of the structure). Pressures can be upwards of 20,000 pounds per square inch (psi), thus failures of the mechanical structure of the BOP can be catastrophic and must be prevented. Transmitting electronic measurements taken from the inside of a pressurized component, such as a BOP, to the outside, is not possible with RF, because RF does not transmit through steel. Also, it is not possible to use a “wired connection” to transmit the readings of a sensor on the inside of the BOP to the outside, because it is not allowed to bore a hole through a wall of the BOP; doing so would compromise its structural integrity and ability of the BOP to withhold extremely high pressures.
The valve(s) of a BOP are closed, either manually or automatically, in order to prevent extremely high-pressure well fluids from blowing out of the well rig, which could otherwise harm the well operators and/or compromise or destroy the rig itself. Thus, it is important to accurately monitor the pressure the BOP is withstanding. One type of BOP, called an annular type, stops the flow of fluids out of the annulus, which is the area between the outside of the drill pipe and the casing of the wellbore. By pumping dense mud into the wellbore, down the drill string, downhole pressure can be overcome. Having accurate measurements of pressures within the BOP would enable operators to know the density of mud that should be used to kill the well and/or know at what point the pressures are safe for possible continued operation or drilling. Having accurate measurements of pressures is also important during testing and maintenance phases of the BOP lifecycle. The requirement of measuring extremely high pressures remains; whether they occur in operation in the field, or in a controlled testing environment. The inventive concept described enables ultrasonic communication mechanisms to transmit pressure readings (and/or other sensor readings) from an ultrasonic communication module mounted on the inside of the BOP, through its thick steel wall, to a receiving module mounted on the outside of the BOP. Once the sensor measurement reading is available on the outside of the BOP, it can be transmitted from the externally mounted module to the desired final destination using traditional (e.g., wired or RF) or ultrasonic communication channels, as appropriate.
In addition to measuring pressures from within a single BOP, the inventive concept also addresses the requirement of communicating pressure readings among BOPs forming a BOP stack. A BOP stack is a structure of two or more BOPs used to control pressure in a well. A typical stack may have different types of BOPs, e.g., annular types at the top and ram types near the bottom of the stack. Communication of pressure readings among a stack of BOPs provides the ability to implement automatic control mechanisms in which pressure readings from all BOPs can be considered in making timely control and actuation decisions. Additionally, ultrasonic communication modules residing on multiple BOPs of a stack can form a relay network for communicating information measured at one location of the stack to another location on the stack, which may serve as a communication gateway for communicating to other components of the rig or locations where operators of the rig can monitor pressure readings and take necessary action.
Accordingly, a need exists in the prior art to obtain information from inside sealed RF-impermeable enclosures, without compromising the enclosure's integrity. To such systems and methods for transmitting information through RF-impermeable sealed enclosures the inventive concept disclosed herein is directed.